There are known many types of switching semiconductor devices as, for example, bipolar transistors, MOS transistors, tunnel diodes, thyristors, amorphous switches, which are used in memory cells. A memory cell is characterized by the fact that it has two stable states, ON and OFF. These states differ by the value of one or more characteristic parameters, this difference being transmitted to the output. The memory cell remains in a determined state even after the triggering signal has ceased, the state parameters having a hysteresis characteristic as a function of the triggering signal for changing states.
Switching time, i.e. the time for changing from one state to the other and the time for triggering to the initial state, is an essential characteristic of the above-mentioned devices and of the memory cells which include them. The switching time is determined by the physical principle of operation and by technological limits established by the device structure and parameters of the materials used.
Among the above-mentioned devices, the bipolar transistor, the MOS transistor and the thyristor can be treated as quadripoles, the triggering signal being applied to the input which is isolated from the output due to presence of a control electrode. The tunnel diode and the ovistor are dipoles and have no control electrode. Switching devices with a control electrode, as compared with those without a control electrode, have the advantage that they need a lower triggering power and that they provide a better isolation (separation) between the input and the output.
Excepting the amorphous switch, which has its own hysteresis loop, the other switching devices have the disadvantage that they can be used in memory cells only in connection with other circuit components.
Also, the described prior devices have the disadvantage that they are characterized by an electrical coupling between the input and the output, due to the electrical interaction among their parts.
All the above-mentioned devices have the disadvantage that they do not fulfill simultaneously all of the requirements needed for a good switching device to be used in a memory cell, namely: an intrinsic hysteresis loop, short switching time, control electrode, and electrical isolation (separation) between the input and the output.
The present invention avoids the above-mentioned disadvantage providing a new class of semiconductor devices which has a basic structure made of a semiconductor material body containing a p-n heterojunction between two p-type and n-type regions; the n-type region is made of an indirect semiconductor material which has an indirect forbidden band gap larger than the direct forbidden band gap and also larger than the indirect band gap of the direct semiconductor material of which the p-type region is made.
The heterojunction exhibits a notch-spike direct-indirect structure. In this heterojunction two mechanisms for electron flow from the n-type region to the p-type region are possible, namely, a slow and a fast flow.
The switching from the slow mechanism to the fast one is done by avalanche filling of the direct notch due to the enhancement of electron-electron interaction. This interaction occurs between indirect electrons injected over the indirect spike and the direct electrons which exist already in the direct notch. The avalanche filling is produced when a critical value of the notch electron population is exceeded.
The switching from the fast mechanism to the slow one is done by emptying of the direct notch due to attenuation of the electron-electron interaction. This interaction takes place between indirect electrons injected over the indirect spike and direct electrons which already exist in the direct notch. The emptying is brought about when the notch electron population decreases below a critical value.
The basic structure is provided with circuit means in order to bias with a variable external voltage the p-n heterojunction coupled with a load. A current-voltage characteristic is produced which has two branches or limbs, the OFF branch and the ON branch. The operating point is on the OFF branch of the current-voltage characteristic. When a critical point ON, determined by a critical voltage level ON and by a critical current level ON is exceeded, these levels being correlated with an increase of the notch electron population over a critical value, the device switches on the ON branch of the current-voltage characteristic.
If the operating point is on the ON branch of the current-voltage characteristic and the bias is decreased, when a critical point OFF, determined by a critical voltage level OFF and a critical current level OFF, is exceeded, these latter levels being correlated with a decrease of notch electron population below a critical value, the device switches on the OFF branch of the current-voltage characteristic. A hysteresis loop is thus obtained which is defined by the two branches of the current-voltage characteristic and by two load lines which pass through the ON and OFF critical points.
The device having this basic structure and employing simple circuitry can be used as a bistable unit with a memory effect.
Starting from the basic bistable memory-effect structure, devices controlled by a control electrode or by optical means, can be made.
The new devices can optically display ON and OFF states. When coupled with photodetectors they permit electrical isolation of the input from the output.
The bistable memory-effect device, with control electrode, can be used for the generation of electromagnetic oscillations.
The devices belonging to the above described class have switching times determined by fast intrinsic processes.